1. Field of the Invention
The present invention relates to an improved cylindrical whole body magnetic resonance imaging (MRI) gradient coil for providing a magnetic field having a substantially linear gradient in an axial direction within the volume of the cylinder, and more particularly, to a cylindrical whole body MRI gradient coil having N windings interleavingly wound in the same direction about the surface of the coil support and separated from each other winding of the N windings by an azimuthal angle of approximately 360.degree./N. The resulting MRI gradient coil provides a magnetic field which is not only linear in the axial direction of the coil but also substantially linear in transverse directions of the coil.
2. Description of the Prior Art
An essential component of current magnetic resonance imaging systems is a winding of wire, called a "coil", whose purpose is to create a linear spatial gradient in the magnetic field within the coil. By convention, the coil is described using x, y and z coordinates, with the z axis normally parallel to the axis of the main magnet for systems in which the magnet has cylindrical geometry, such as for whole body imaging. As known to those skilled in the art, two kinds of coils are needed, namely, one which creates a gradient along the z (or longitudinal) axis of the coil, and two others which create gradients along either the x or y (transverse) axes. The present invention is primarily directed to an improved z gradient coil.
As known to those skilled in the art, the desired magnetic field gradients can in principle be created by a suitable pattern of current flowing on the surface of the cylinder of a cylindrical coil. However, if a small number of loops of wire are used to create the field, the resultant gradient suffers from inhomogeneity, meaning that the gradient is not optimally uniform over the desired region of space. Such principles are well known and have been addressed by Schenck in U.S. Pat. No. 4,617,516. In that patent, Schenck discloses a z gradient coil of the type illustrated in FIG. 1. As shown, Schenck's z gradient coil comprises a single wire 12 wrapped around a coil support structure 10 in a helical path such that the spacing between respective turns decreases in proportion to the distance from the center of the coil. In this single wire design, the center of the coil is anomalous because the winding changes its winding direction at center point 14.
As shown in FIG. 1, electrically conductive winding turns 12 are wound on the outer surface of cylindrical support structure 10 so that the axial density of the winding turns 12 is at a minimum at the center of the length of the coil support structure 10 and increases linearly from the center to each axial end of the coil. Winding turns 12 are wound so that they are symmetrically located about the center of coil support structure 10. At the center point 14, the direction of winding turns 12 is reversed so that the winding turns 12 are symmetrically located about the center point 14 of support structure 10. Winding turns 12 are also electrically insulated from each other in order to provide the desired current paths around the outer surface of cylindrical coil support structure 10. Terminal connections 16 are also used to connect the winding turns 12 with a power supply (not shown) which supplies a current to the winding turns 12. Generally, winding turns 12 are made from any electrically conductive material such as copper, while the coil support structure 10 is made from any non-magnetic electrically insulating material having sufficient strength and rigidity to be formed into a cylinder of the size required for a particular application. For example, coil support structure 10 may be comprised of glass fiber material. Further details regarding the single wire design of prior art FIG. 1 can be found in the aforementioned patent to Schenck.
While the single wire design of Schenck has been found to provide a substantially linear gradient in the axial direction within the volume of a cylindrical whole body z gradient coil, the single wire design is limited because of the tradeoffs implicit in choosing the density of the turns. For example, if the density is too low, then near the center of the coil there will be unwanted inhomogeneity of the gradient field. In particular, the field will exhibit an unwanted transverse dependence, becoming stronger as one approaches the side which contains the wire and weaker on the opposite side. This effect can be reduced, but not eliminated, by increasing the winding density so that there are more turns per inch at each point in the coil. However, this approach has the disadvantage that the increased turns increase the resistance and inductance of the coil, which is undesirable because the coil will then require a higher voltage driving circuit to achieve acceptably fast switching of the current pulses as needed for MRI imaging.
Accordingly, an improved z gradient coil is desired which eliminates the unwanted transverse dependence of the single wire z gradient coil without undesirably increasing the inductance and resistance of the coil. The present invention has been designed to meet these needs.